The middle layer of the world's oceans receives only faint, filtered sunlight during the daytime. This is because the seawater absorbs the sunlight. This barely-lit ocean layer is called the twilight zone or the disphotic zone (disphotic means "poorly lit" in Greek) or the mesopelagic zone (mesopelagic means "middle sea"). This zone appears deep blue to black in color. The depth of this zone depends on the clarity or murkiness of the water. In clear water, the disphotic zone can begin at depths up to 600 feet; in murky water, it can start at only 50 feet deep. It is usually begins somewhere between these two extremes. The disphotic zone extends to about 3,300 feet (about 1,000 m) deep (this is where the aphotic zone begins). On average, this zone extends from 660 to 3,300 feet (200 to 1,000 m).

In the disphotic zone, there is enough light to see during the day, but not enough light for photosynthesis to take place, so no plants live in this zone. The amout of light decreases with depth. Because of this, food is not abundant.

The water in the disphotic zone is cold (the temperature ranges from 41 to 39 degrees F) and decreases with depth. The pressure is high -- it can be up to 1,470 psi (pounds per square inch) and increases with depth. The amount of dissolved oxygen in the water is less than in the sunlit zone.

Animals:The animals that live in the disphotic zone are adapted to life in near darkness, cold water and high pressure. Many of the animals in this zone have large eyes, helping them see in the nearly dark waters. Most are small, dark and thin (to help camouflage them). Many have large teeth and jaws.

Bioluminescence: Many of the animals in the disphotic zone are bioluminescent; they can make their own light. Some bioluminescing animals have special organs that produce light by a chemical reaction; other bioluminescing aniamls have glowing bacteria that live on them. Most bioluminesscing organs called photophores give off an eerie blue-green light. A rare exception is the Malacosteidae or Loosejaw fish, who produce a red light for intraspecies communication (red is invisible to most other deep-sea marine organisms, who see mainly blues and greens). Animals use their light to help them find food, to help them find mates, and/or to confuse predators (they do this by camouflaging themselves or distracting predators).

Counterillumination is a method of camouflage. Lght produced on the underside of bioluminescing animals can help the animal "disappear" from predators that are below it. The top part of the animal is not lit; to animals looking down from above the prey, the prey animal is virtually invisible since its dark silhouette appears against a black background.

Diet: Animals in this zone feed on plant matter and algae that fall into this zone from the euphotic zone (the well-lit zone above the disphotic zone), animals that stray into this twilight zone, and each other. Animals in the disphotic zone are filter feeders, grazers, and predators. Some animals migrate vertically (up and down) in order to feed upon the abundant life in the bright zone above them. Most of the animals that eat other animals in this zone do not chase their prey; they lure the prey to them or simply wait for the prey to come to them. Some predators lure their prey with bioluminescent barbels (fleshy projections near the mouth) that look like small animals.

CoelacanthThe Coelacanth (pronounced SEE-la-canth) is a primitive lobe-finned fish that was thought to have been extinct for millions of years, but a living Coelacanth was caught in the Indian Ocean off the coast of South Africa in 1938.